As a jet engine of a flying body to be flying faster than a speed of sound, a turbojet engine (including a turbofan engine etc.), a ramjet engine and a scramjet engine are known. These are jet engines, and in particular in the ramjet engine and the scramjet engine, a speed of air taken therein depends strongly on a flight speed.
FIG. 1 and FIG. 2 are schematic cross-sectional views schematically indicating a configuration of a jet engine combustor. The combustor includes a fuel injector and an igniter. The fuel injector is formed in a wall of the combustor. The fuel injector injects fuel toward a space 150. The igniter emits flame toward a space 150 (FIG. 1). The flame generated by the igniter ignites a gas mixture of a mainstream of air and the fuel. Flame generated by the ignition of the gas mixture propagates to a flame holder, and the flame is kept in the flame holder. The flame kept in the flame holder burns the gas mixture of the mainstream of air and the fuel (FIG. 2). Then, combustion gas is discharged from a nozzle arranged rearward of the combustor, and the jet engine obtains thrust.
As a method for igniting the gas mixture, a method of installing a small type solid rocket motor igniter in a deep recess etc. formed in a wall surface of the combustor and igniting the gas mixture using the flame generated by the igniter is known (FIG. 1). In igniting, for vigorously emitting the flame generated by the igniter toward the space as well as for increasing a filling ratio of the solid fuel, a shape having deep configuration is adopted as a shape of the recess for installing the igniter. In general, when a maximum length of the recess along a direction of the mainstream of the air is defined as a length l, and a maximum depth of the recess is defined as a depth d, the length l and the depth d are set such that l<d is satisfied.
The flame generated by the igniter propagates along a direction indicated by an arrow 160 by burning the gas mixture. Then, the propagated flame is kept in the flame holder.
As a flame holding mechanism, a method of forming a low speed region of the gas mixture using a shallow depression etc. formed in the wall surface of the combustor and keeping the flame using the low speed region is known (FIG. 2). In general, when a maximum length of the depression along the direction of the mainstream of the air is defined as a length L, and a maximum depth of the depression is defined as a depth D, the length L and the depth D are set such that L>D is satisfied.
As a related technology, in Japanese Patent Literature JP H06-060597 B2, a method of igniting for a combustor and keeping flame is disclosed.